1. Field of the Invention
The present invention pertains to wireless telecommunications, and particularly to inter-radio access technology handover for wireless telecommunications.
2. Related Art and Other Considerations
Some mobile terminals are capable of using more than one type of radio access technology (RAT) in order to communicate with, e.g., other mobile terminals or one or more core networks. Such mobile terminals have been termed as “dual mode” mobile terminals in view of their capability to use (e.g., at differing times) at least two radio access technology types. Each radio access technology type is implemented using an associated radio access subsystem or network. Two examples of differing radio access technologies are (1) the Global System for Mobile communications (GSM) developed in Europe, and its third generation successor (2) the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). UTRAN is essentially a wideband code division multiple access (W-CDMA) system.
An inter-radio access technology (RAT) handover is process wherein a mobile terminal switches from using a first radio access system having a first radio access technology (such as GSM) to a second radio access system having a second radio access technology (such as UTRA). Inter-RAT handover is normally initiated when the quality of a downlink radio connection of the first radio access network falls below a certain level. The quality of the uplink connection with the first radio access network may be considerably poorer than the downlink quality.
In the inter-RAT procedure, a handover command message is sent to the mobile terminal to provide the mobile terminal with details of the radio resources to be used after the handover (e.g., when the mobile terminal is in communication with the second radio access network using the second radio access technology). As an example, consider a situation in which the first radio access network with which a mobile terminal has been communicating uses GSM radio access technology (RAT), and wherein measurements warrant that the dual mode mobile terminal change to use UTRA radio access technology. In such situation, the handover command message that is sent by a base station controller of the GSM radio access network across the GSM air interface to the mobile station comprises one or more segments of limited length (each segment being twenty one octets). When a segment of twenty one octets is insufficient to transmit the radio resource information necessary for the handover, the handover command message must be segmented to comprise plural segments. However, after a segment of the handover command message is sent, a subsequent segment cannot be transferred until receipt of the preceding segment has been acknowledged.
The handover command message is sent from the base station controller to the mobile terminal via the downlink radio connection. However, in a case in which the handover command message is segmented, successful transfer of the handover command message requires that acknowledgement of a preceding segment be transferred via the uplink radio connection prior to transmission of a further segment. Thus, if segmentation is used, it may be impossible to quickly transfer the handover command message due to poor quality of the uplink radio connection. In other words, segmentation of the handover command message over more than two GSM air interface messages can have a significantly detrimental, and unacceptable, impact on handover performance.
In the situation of handover from GSM to UTRAN, the GSM handover command message encompasses a RRC Handover to UTRAN command message. The RRC Handover to UTRAN command message includes the details of the UTRA radio resources to be used in the UTRAN after handover. The RRC Handover to UTRAN command message includes a large number of parameters stored in respective information elements of the RRC Handover to UTRAN command message. The size of the RRC Handover to UTRAN command message depends on the actual content of these parameters, and can be as long as one hundred to two hundred octets. When the RRC Handover to UTRAN command message is so large, it obviously cannot fit within a non-segmented GSM air interface message, with the result that the RRC Handover to UTRAN command message must be segmented.
Various techniques have been utilized as attempts to reduced the size of the handover command message, and thus to avoid segmentation of the handover command message. As a first example technique is preconfiguration. There are two basic types of preconfiguration. A first type of preconfiguration is pre-defined configuration. In pre-defined configuration, a network can prepare and download one or more radio configurations to a mobile terminal. Such a pre-defined radio configuration comprises a large number of radio bearer parameters, transport channel parameters, and physical channel parameters. Prior to RAT handover, the UTRAN inquires which configurations are stored in the mobile terminal. In case the mobile terminal has suitable pre-defined configurations stored thereat, the UTRAN can refer to a stored configuration and then only needs to signal a few additional parameters to be used in addition to the stored parameters associated with the pre-defined configuration.
A second type of preconfiguration is default parameter configuration. In default parameter configuration, a number of parameters have default values which are specified in a standard, such as RRC Protocol Specification TS 25.331. The default parameter configuration can be utilized in a similar manner, but since it does not need to be downloaded, it can be used more easily.
As a second example technique for reducing length of the handover command message, several initially non-essential parameters can be omitted from the handover command message. It is assumed by this technique, however, that the corresponding functionality of these parameters need not be configured immediately upon handover.
As a third example technique, for some of the parameters transferred when using preconfiguation, the use of smaller value ranges for the parameters can be predetermined. In other words, the sizes of certain information elements in the handover command message is reduced by reserving a subrange of the information element parameter values for exclusive use by the mobile terminal which is performing the inter-RAT handover. An explanation of this third technique is provided in U.S. patent application Ser. No. 09/483,743, filed Jan. 17, 2000, entitled “Method and System For Improving The Performance of Inter-System Handovers”, which is incorporated herein by reference in its entirety. This third technique helps to reduce further the size of the handover command message, although sometimes there is a slight performance degradation.
Despite the various techniques that have been used to reduce the size of handover command messages, the handover command message nevertheless approaches the limit imposed by the space available in a non-segmented GSM air interface message. Unless other mechanisms are defined, hardly any space is left in the RRC Handover to UTRAN command message to facilitate the inclusion of further parameters.
One of the parameters that needs to be included in the RRC Handover to UTRAN command message, when using preconfiguration, is a parameter having an information element known as “Default DPCH Offset Value”. Persons skilled in the art will appreciate that DPCH refers to a dedicated physical channel which corresponds to a spreading code (see, e.g., 3GPP TS 25.211, v.3.2.0 “Physical Channels and Mapping of Transport Channels Onto Physical Channels (FDD)” or 3GPP TS 25.221, v.3.2.0 “Physical Channels and Mapping of Transport Channels Onto Physical Channels (TDD)”). The Default DPCH Offset Value parameter is used to distribute evenly the processing and transmission load of different mobile terminals over time. The time interval over which the distribution is performed is 80 milliseconds, which corresponds with the maximum transmission time interval (TTI). Adding the Default DPCH Offset Value parameter to the RRC Handover to UTRAN command message would introduce an additional ten bits (600 steps of 512 chips). As indicated before, this is undesirable since the size of the RRC Handover to UTRAN command message is already close to the limit for a non-segmented GSM air interface message.
What is needed, therefore, and an object of the present invention, is a technique for facilitating transmission of additional parameters in a handover command message in conjunction with inter-RAT handover without appreciably lengthening the handover command message.